A system includes optical discs, having an optically transparent substrate and an optically scannable information layer present on said substrate; and a scanning device for scanning the optical discs. The scanning device includes a turntable having a supporting surface for carrying optical disc and a scanning unit comprising an actuator and an objective. The objective has an optical axis for focusing a radiation beam passing through the substrate of an optical disc situated on the turntable so as to form a scanning spot on the information surface. The actuator comprises a first actuator section and a second actuator section including the objective. The second actuator section, for the purpose of focusing, is movable relative to the first actuator section in directions parallel to the optical axis.
Such a system is generally known in the form of the standardized "Compact Disc" system. This known system comprises optical discs--compact discs--having an information layer with a spiral track comprising a succession of pits and ridges. The information is stored in digital form in the track of a compact disc, the pits and ridges representing series of bits. The digital information on the compact disc is protected by a transparent substrate having a nominal thickness of 1.2 mm.
The "Compact Disc" system further includes players with a scanning device for the optical scanning of a compact disc. Such a scanning device is disclosed in, for example, EP-A 0 464 912 (U.S. Pat. No. 5,199,014). The scanning device comprises a turntable for supporting, centering and rotating a compact disc and further comprises an electro-optical scanning unit with an objective and an actuator for moving the lens. Optical scanning is effected by means of a solid-state laser source, the beam of laser light being focused on the information layer of the compact disc via the objective, the substrate being disposed between the objective and the information layer. Since the digital information on the compact disc is shielded by the substrate possible flaws and dust particles are not situated in the focal plane of the light beam which scans the disc, as a result they have comparatively little influence.
In order to achieve a correct focusing of the light beam, the actuator comprises a stationary actuator section and a movable actuator section and it comprises a drive assembly of a coil and a permanent magnet to move the objective in the direction of its optical axis. During focusing the movable actuator section performs translatory movements about a center position. A focus servo system provides the drive of the coil. In the course of time various types of optical discs, based on the "Compact Disc" standard, have been developed, such as CD-ROM, CD-I, CD-R, which all have a substrate of a thickness in conformity with the value specified in the standard. Recently, a digital video disc (DVD) has been proposed, which comprises two transparent substrates between which information layers are disposed. The substrates of said video disc each have a nominal thickness of 0.6 mm. Owing to the deviating substrate thickness this type of optical disc cannot readily be scanned by means of the scanning device of the known system.